I18n localize - compile-time - #32105

The missing translation and invalid placeholder warnings now contain the
"meaning" if one was provided.

Previously the metadata and placeholder blocks were serialized in
a variety of places. Moreover the code for creating the `LocalizedString`
AST node was doing serialization, which break the separation of concerns.

Now this is all done by the code that renders the AST and is refactored into
helper functions to avoid repeating the behaviour.

Metadata blocks are delimited by colons. Previously the code naively just
looked for the next colon in the string as the end marker.

This commit supports escaping colons within the metadata content.
The Angular compiler has been updated to add escaping as required.

In an attempt to be compatible with previous translation files
the Angular compiler was generating instructions that always
included the message id. This was because it was not possible
to accurately re-generate the id from the calls to `$localize()` alone.

In line with https://hackmd.io/EQF4_-atSXK4XWg8eAha2g this
commit changes the compiler so that it only renders ids if they are
"custom" ones provided by the template author.

BREAKING CHANGE:

When translating messages generated by the Angular compiler
from i18n tags in templates, the `$localize.translate()` function
will compute message ids, if no custom id is provided, using a
common digest function that only relies upon the information
available in the `$localize()` calls.

This computed message id will not be the same as the message
ids stored in legacy translation files. Such files will need to be
migrated to use the new common digest function.

This only affects developers who have been trialling `$localize`, have
been calling `loadTranslations()`, and are not exclusively using custom
ids in their templates.

This commit provides the main pieces for inlining translations at compile-time.